1. Field of the Invention
The present invention relates to a terahertz wave optical device such as a terahertz wave generating device for generating a terahertz wave by irradiation with excitation light. In addition, the present invention relates to an apparatus such as a terahertz wave generating apparatus that uses the terahertz wave generating device.
2. Description of the Related Art
The terahertz wave is an electromagnetic wave having an arbitrary frequency band in the range from 0.03 THz to 30 THz. In the terahertz wave band, there are many distinctive absorption regions depending on structures and states of various substances such as a biomolecule. There is developed an inspection technology of performing analysis or identification of a substance while the substance is not destroyed utilizing the characteristic described above. In addition, the terahertz wave is expected to be applied to a safe imaging technology without using x-rays and a high speed communication technology.
In order to implement those technologies, it is one of important elements to advance development of a technology of generating a terahertz wave. In recent years, as the technology of generating a terahertz wave, a terahertz wave generating device utilizing a PIN photodiode is disclosed in Appl. Phys. Lett. 59, 3357 (1991). This terahertz wave generating device has a structure in which electrodes are disposed so as to sandwich a PIN photodiode layer serving as a carrier generating portion, whereby an electric field is applied in a thickness direction of the PIN photodiode layer. Then, a pulse beam is irradiated in a slanting direction with respect to the application direction of the electric field, and hence a terahertz wave is generated. The intensity of the terahertz wave depends on the intensity of the electric field that is applied to the generated carrier. With this device structure, the film thickness of the carrier generating portion is adjusted so as to adjust a distance between the electrodes. As a result, a stronger electric field can be easily applied to the carrier, and hence a high intensity terahertz wave generating device may be provided
The terahertz wave is generated by a dipole radiation accompanying migration of the carrier generated abruptly by irradiation with the excitation light. On this occasion, there is no component of the electric field or the magnetic field of the terahertz wave on an extension line in the dipole direction, and hence it is difficult to radiate the terahertz wave in the application direction of the electric field. Therefore, in the conventional terahertz wave generating device such as the device disclosed in Appl. Phys. Lett. 59, 3357 (1991), in which the electric field is applied in the thickness direction of the carrier generating portion, a method has been widely adopted, in which the excitation light is irradiated in a slanting direction to the terahertz wave generating device. In addition, it is known that an incident angle of the excitation light also depends on the device structure and that there is an angle at which maximum radiation efficiency can be obtained. From those facts, the step for adjusting the incident angle of the excitation light irradiating the terahertz wave generating device becomes necessary, and hence an optical adjustment may become complicated. Depending on an apparatus form to be provided, it may be required to improve such complicated adjustment.